Comparing IS barrier solutions

Significant savings, both initial installation and ongoing maintenance costs, for an intrinsically safe (IS) facility or project can be achieved by selecting the appropriate apparatus as the IS barrier for the system.

Preventing explosions and fires in hazardous areas caused by process measurement and control instrumentation has historically followed either the path of containing the explosion within the device enclosure or of preventing the device from having enough energy to cause a spark or thermal ignition. In many places, such as the United States, the predominant choice has been to use explosion-proof equipment, while many other parts of the world typically employ energy-limiting intrinsic safety devices.

Although there has been some resistance to change from those using the familiar explosion-proof approach, engineers have recognised the cost savings and advantages of an IS design, leading to wider acceptance of IS. Additionally, globalisation of many corporate structures often leads to standardisation of plant designs that are the best economic fit on a global basis and these designs frequently require use of IS technology.

This article provides a brief introduction to intrinsic safety, the different components in an IS system and the two different types of barriers. Additionally, this article outlines why selecting an associated apparatus as the IS barrier provides the most economic and effective use of IS technology. The techniques outlined in this article are most applicable to the industrial process control sector including such industries as oil and gas production, oil refining, petrochemical, chemical, pharmaceuticals, food and beverage, and pulp and paper.

The concept of intrinsic safety

Instead of using explosion-proof techniques to contain a possible explosion, the IS approach limits the electrical and thermal energy that could reach any device in the hazardous area. This ensures that the energy level remains below threshold levels that would ignite an explosive atmosphere. The vast majority of field instrumentation devices, such as transmitters and solenoid valves, typically operate on 24 VDC or less with low current signal levels that are well within typical IS system limits.

There are a number of approval agencies that certify IS devices including FM, CSA International, SIRA, LCIE, Testsafe and many others who offer North American, ATEX and IEC Ex based certifications for gas, dust and fibre hazardous environments. An IS system includes the field devices, the barriers and/or the associated IS devices, and the interconnecting cable (see Figure 1).

Figure 1: Components of a typical intrinsically safe system. For a larger image, click here.

Field device IS classifications

Simple apparatus include devices such as RTDs, thermocouples, switches, LEDs, potentiometers and switches. They are electrical components that do not generate or store more than 1.5 V, 100 mA and 25 mW, or a passive component that does not dissipate more than 1.3 W. Simple devices can be freely used without any agency certifications but do require an assessment for their maximum surface temperature and to be assigned a temperature classification (referred to as a T code).

Intrinsically safe apparatus are devices that can store electrical energy such as transmitters, I/P converters and solenoid valves. They may also be connected to simple apparatus in the hazardous field location. These devices must be certified as intrinsically safe apparatus and classified based on allowable hazardous locations, gas group and T code. Entity parameters for the device must also be provided and include the maximum voltage, current and power limits as well as the internal capacitance and inductance parameters of the device. These parameters are used in conjunction with the connecting cable parameters to calculate the maximum allowable cable lengths, loop voltage and current values for the system.

Barrier or associated apparatus

An IS system installation requires a barrier or associated apparatus interface between the field device and the control room equipment. Its function is to limit the energy to the hazardous area such that, even under a fault condition, there cannot be enough electrical or thermal energy released by the device to ignite an explosive atmosphere. They are designed for connection to simple or IS apparatus and must be certified. There are two types of barriers that are most commonly used and a hybrid method where the barrier is integrated into the receiving device.

Zener diode barriers are simple passive devices comprised of Zener diodes, resistors and fuses that serve to limit the voltage, current and power available to the hazardous area device. The design requires the use of a dedicated IS earth ground connection maintained at less than 1Ω and allows no grounding connections at the field devices. A common downside of using this approach is that the required earth ground has low noise rejection capability. This electrical interference can introduce stray and unwanted electrical noise components into the measurement circuit, creating potentially significant measurement errors.

Figure 2: A system using a safe area device with an external barrier and power supply. For a larger image, click here.

Isolated barriers are active devices that incorporate galvanic isolation, thus eliminating the requirement for an earth ground and the restriction for grounding of field devices. They also provide a higher voltage to the field and devices. These barriers require operating power and are application specific, with different models required for different applications (RTD, thermocouple, 4-20 mA etc).

Associated apparatus incorporate a barrier into the safe area mounted receiving device or the control room equipment. The Moore Industries SPA2IS is an example of such a device that provides an isolating barrier within the alarm trip. This dramatically reduces the cost of purchase, installation and maintenance versus more traditional approaches that require a separate Zener or isolating barrier (see Figure 3).

Figure 3: An associated apparatus incorporating the isolating barrier, temperature transmitter, temperature alarm and diagnostic alarming functions in a single device. For a larger image, click here.

Design considerations

As discussed, the premise of an intrinsically safe system is that there is no component or combination of components that can release enough electrical or thermal energy to ignite an explosion in the hazardous area either under normal or fault conditions. In order to accomplish this goal, the energy storage and release characteristics of all components must be defined and incorporated into the system design.

While this may sound like a daunting task, it is relatively simple in practice. The manufacturer of each component must provide a certification document (or data sheet) that lists the definitive voltage, current, power, inductive and capacitive values appropriate to the application. These are called ‘entity parameters’. As an example, the capacitance of the field-mounted transmitter and its output cable must not exceed the allowable value specified by the associated device (barrier) in the safe area. This is a simple A + B ≤ C calculation for the capacitance (C) and the inductance (L) of the transmitter and the cable. And further, the output voltage of the barrier must be less than the maximum allowed by the transmitter and similarly the output current of the barrier must be less than that allowed by the transmitter.

The combined values of capacitance and inductance for a typical transmitter and 400 m of cable are far less than the maximum allowable by a typical barrier or associated IS device. The voltage, current and power specification of a typical associated IS device (barrier) is limited by vendor design to acceptable numbers for the intended application. For a transmitter barrier for example, the maximum voltage is typically less than 30 VDC, and the maximum current is less than 100 mA.

Table 1: Design constraints associated with entity parameters (for a larger image click here).

To certify the installation, a system assessment document is created based on the entity parameters of each component and a verification performed to ensure that all values of the system are within the allowable limits.

Installation and maintenance considerations

One advantage of IS installations is that due to the low power, ordinary instrument cables can be used for IS circuits. Maintenance and calibration of field equipment can also be carried out while the plant is in operation and the circuit is ‘live’ in the hazardous area.

A key design decision, which can have a significant effect on the IS system installation and maintenance costs, is the choice of barriers. While Zener barriers are less expensive than active isolated barriers, they require a separate, clean, high integrity ground, which has high maintenance costs and potential for electrical noise issues. An isolated barrier is often the better choice but cost, maintenance and cabinet space of barrier power supplies need to be included. This may also involve redundant systems, since power supplies usually have the highest failure rate and can significantly reduce system reliability. This further adds to required cabinet space and heat dissipation or cooling considerations in your barrier marshalling cabinets. Often the additional cost of the isolated barriers and power supplies are more than the field-mount instruments themselves.

An often-overlooked consideration is the use of associated apparatus. These offer the dual role of transmitter and isolated barrier in one package, which can provide significant cost savings by reducing the number of components, power supply requirements, cabinet space, wiring terminations, installation labour and stocking requirements. Cost savings are ongoing with reduced spares inventory, maintenance-related downtime and consequent process restart issues.

Conclusion

Intrinsically safe systems are becoming more prevalent in the process control industry and offer some advantages over explosion-proof systems when used for field instrumentation. Since the energy is limited, general-purpose wiring methods can be used (no rigid conduit, pouring of seals or special housings are needed). Also, equipment can be replaced and maintained without having to un-power loops or shut down the process.

However, a disadvantage is the installation and maintenance costs of the required IS barriers. Many, but of course not all, of these costs can be drastically reduced if an associated apparatus is used. Since the associated apparatus includes the barrier in the receiving device there is no need for the additional cost of the barrier, cabinet space, a high-integrity clean ground connection, separate power supply or custom vendor backplane.

The associated apparatus provides an integral solution that is the most affordable and safe IS solution available.

References

1. Instrumentation, Systems, and Automation Society 2003, ANSI/ISA-RP12.06.01-2003 Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation Part 1: Intrinsic Safety.
2. Instrumentation, Systems, and Automation Society 2009, ANSI/ISA 60079-11 (12.02.01)-2009 Explosive Atmospheres — Part 11: Equipment protection by intrinsic safety “i”.
3. International Electrotechnical Commission 2015, IEC 60079-10-1:2015 Explosive atmospheres — Part 10-1: Classification of areas - Explosive gas atmospheres.
4. International Electrotechnical Commission 2011, IEC 60079-11:2011 Explosive atmospheres — Part 11: Equipment protection by intrinsic safety “i”.
5. Factory Mutual Research Corporation 2015, FM 3610:2015 Intrinsically Safe Apparatus and Associated Apparatus For Use In Class 1, 2 And 3, Division 1, Hazardous (Classified) Locations.

*TS Todd is Director of Engineering at Moore Industries. She has a BSEE from Brunel University and more than 25 years of systems engineering experience in industrial, communications and aerospace applications.

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